This invention relates generally to high frequency signal processing devices and, more particularly, to devices which utilize bulk acoustic waves for such processing.
Various devices are used to process signals at radio frequencies. Some devices use surface acoustic waves (hereinafter sometimes called SAW) for analog signal processing, such as filtering, spectrum analyzing, correlation, convolution, and pulse compression. The SAW devices are relatively simple and inexpensive to fabricate, and they have the advantages of being both passive and compact. A typical device is described, for example, in U.S. Pat. No. 4,101,965, which issued on July 18, 1978 to K. Ingebrigtsen et al.
Because of the low velocity of surface acoustic waves along the surface of a crystal, excessively long interaction times between the received signals and the apparatus are accommodated on a short substrate. Bandwidths of up to about 200 Megahertz (MHz) are achieved. Such bandwidth and interaction time characteristics make SAW devices highly useful as signal processing devices. However, the bandwidth of a SAW device is fixed at about 20% of its band center frequency. To increase the bandwidth of a SAW device, the center frequency must be increased. But, because the acoustic wave is dispersed as it travels along the crystal surface, the center frequencies of existing SAW devices have a practical upper limit of about 1 GHz, and its bandwidth is limited to about 200 MHz. SAW propagation can often be susceptible to surface damage and attenuation by the crystal material. Such attenuation increases as the square of the frequency.
Advances in miniaturizing electronic circuits produce no significant decreases in weight and volume of SAW devices.
Because of the frequency limitations of SAW devices, use of microwave superconducting strip transmission lines has been suggested, for example, in the article "Passive Superconducting Microwave Circuits For 2-20 GHz Bandwidth Analog Signal Processing" by J. T. Lynch et al., Proceeding of the MTT Symposium, 1982. Such superconducting devices, however, currently operate only at extremely low temperatures (down to as low as about 4.degree. K.), and require a multi-stage refrigeration system which substantially increases the weight, volume and cost of the overall device and requires significant power to operate. Moreover, they have a time delay, in achieving signal processing operation, after the refrigeration system has been made operative. Superconducting strip lines typically have more manufacturing defects than SAW devices, because fabricating superconducting strip lines is extremely difficult and complex than fabricating SAW devices.
In operation, the high velocity of electromagnetic waves in a superconducting strip transmission line, compared to the velocity of SAW waves, severely reduces the achievable interaction time between the acoustic signal and apparatus of the same path length.
It is important to achieve high frequency operation without the disadvantages of either the SAW device or the microwave superconducting strip transmission device, and without a need for refrigeration, at reasonable, size, cost and with a reasonable ease of fabrication. To that end, bulk acoustic wave (hereinafter sometimes called BAW) apparatus have been conceived.
Bulk acoustic waves are used in a body which supports acoustic transmission through its interior, (hereinafter called BAW body) rather than on its surface. A part of the interior of the BAW body may be processed to have a plurality of regions which have varying acoustic impedance characteristics. The BAW signal interacts with such regions, for example by reflection and refraction, and the apparatus may be designed to process the signals delivered to the BAW apparatus. Acoustical Bulk Wave Processing Devices may, for example, use a reflecting surface which may be curved or flat. It may also use diffraction gratings that steer and focus or spread acoustic waves. See U.S. Pat. No. 4,609,890, which issued Sept. 2, 1986 to Oates et al for "Bulk Acoustic Wave Signal Processing Devices."
Diffraction gratings may be used with BAW devices to steer the acoustic beam. A diffraction grating may be inscribed, etched, or otherwise placed on the surface of the BAW body. Preferably, because of the needed close line spacing, it may be formed by a computer controlled holographic process to control the dispersed acoustic beam and suppress higher diffraction orders. A typical grating, and process for making it, are taught and shown in U.S. Pat. No. 4,547,037 which issued Oct. 15, 1985 to Steven K. Case for a "Holographic Method for Producing Desired Wavefront Transformations."
The profile of the lands and grooves of an efficient diffraction grating may be of a number of shapes. One preferred embodiment of such a grating has a sinusoidal profile. Another embodiment has a rectangular profile. Still another embodiment has a saw-toothed profile.
A diffraction grating having a rectangular profile, and the dispersion of an acoustic or sonic wave is shown in U.S. Pat. No. 4,329,876 which issued to W. H. Chen and E. G. Lean on May 18, 1982 for a "Method and Apparatus for Acoustic Scanning Using Scattering of Bulk Waves by an Acoustic Grating".
The signal density and the number of strange or exotically configured signals are increasing. To achieve a high probability of intercept and identification such signals requires special receivers with very wide instantaneous radio frequency bandwidth, and high sensitivity over a wide dynamic range of received signal intensity.
Crystal video, superheterodyne, instantaneous frequency measurement, channelized, compressive, and Bragg Cell receivers are most commonly used. Channelized receivers are among the best performing of these receivers.
A channelized receiver delivers a radio frequency input signal, preferably amplified, to a first bank of bandpass filters, with their input terminals in parallel and with slightly overlapping passband boundaries between adjacent filters. Each filter is designed to pass signals within a predetermined passband of frequencies.
The first set of filters divides the input frequency spectrum into overlapping frequency "bands". Each filter of the first set of filters delivers its band of signals into the parallel input terminals of a second set of bandpass filters which further divide those bands into a plurality of slightly overlapping frequency "channels". Each filter of each of the second sets of filters delivers its channel of signals into the parallel input terminals of a third set of bandpass filters which further divide those channels into a plurality of overlapping frequency "slots".
The processing of multiple simultaneously-received signals at different frequencies over a wide bandwidth allows the isolation of weak signals into different frequency slots for further analysis while rejecting other uninteresting, known, or jamming signals. Separating the signals on a real-time scale facilitates detection and analysis of frequency-hopping signals and the reception of communications on time-varying, multiple-frequency channels. Compared to other mentioned types of apparatus, a channelized receiver has a high probability-of-intercepting pulsed radio frequency signals and a short acquisition time.
Because of the large quantity of data which may flow through a channelized receiver, it needs a large signal processor. High-speed programs and circuits, together with a large amount of power are needed.